Fuel cells are considered to be low-emission alternatives to conventional energy generating processes which point the way to the future. The polymer electrolyte membrane (PEM) fuel cell is of particular interest for mobile applications. A proton-conducting polymer membrane is the central component in this type of fuel cell.
Numerous studies have been carried out on the use of other polymers as membrane materials in fuel cells. However, these polymers are virtually exclusively sulfonated materials whose proton conductivity is attributable to sulfonic acid groups.
PEM fuel cells comprise two electrodes which are separated from one another by a proton-conducting membrane (polymer electrolyte membrane or proton exchange membrane). The electrodes comprise, for example, carbon mats onto which platinum has been deposited by vapor deposition and which are connected to one another via an external electric circuit. For reaction of hydrogen and oxygen to form water to be able to occur, the proton-conducting membrane has to be moistened. The fuel hydrogen is continuously supplied to the anode. The cathode is continually supplied with oxygen. Two types of PEM fuel cells are being developed: low-temperature cells (up to about 90° C.) and high-temperature cells (up to about 180° C.).
The low-temperature cell was developed in the 1960s. Then, a sulfonated polystyrene membrane served as electrolyte. Since 1969, the Nafion® membrane developed by DuPont has been installed in PEM fuel cells. Low-temperature cells are sensitive to carbon monoxide (CO). This gas can block the anode catalyst, which leads to a decrease in power. The membrane has to be moistened for it to be able to conduct protons.
High-temperature cells are insensitive to CO and other impurities. Higher working temperatures in the cell are beneficial to energy management, since they enable more efficient use of the heat produced. Since the membrane conducts protons without water, it does not need to be moistened.
In PEM (polymer electrolyte membrane) fuel cells, the electrolyte which comprises an ion-conducting polymer membrane is the central component of the cell. The requirements which this membrane has to meet are multifaceted and complex: electrochemical and mechanical stability under cell conditions, processability, high ion conductivity and low permeation of the reactants (hydrogen, methanol, oxygen) have to be combined. A polymer electrolyte membrane which meets all these requirements and is also available at a low price does not exist at present.
About 30 years ago, copolymers of tetrafluoroethylene which had been ionically functionalized by sulfonic acid groups were developed for chloralkali electrolysis. These are still the present-day standard polyelectrolytes for fuel cells. The best known and most widely used representative of these polymers is Nafion®, developed and produced by DuPont. The perfluoroalkylenesulfonic acid polymer is sulfonated and accordingly has excellent proton conductivity. The mechanical and electrochemical stability means that Nafion® is suitable as cell membrane.
However, production of the membrane is difficult and expensive. Proton conduction is accompanied with unwanted diffusion of water in Nafion®. If Nafion® is swollen in water, a high ion conductivity is observed. The applications above 100° C. are therefore not possible. However, higher temperatures are desirable because of the sensitivity of the platinum catalysts used to carbon monoxide (CO) at temperatures below 100° C. In practical applications, the hydrogen used as fuel gas is contaminated with traces of CO. This carbon monoxide (CO) represents a great problem for low-temperature fuel cells since it is adsorbed on the platinum surface and thus poisons the catalyst.
During the course of the search for higher efficiencies of primary energy carriers, proton-conducting PEMs have attained increasing importance in the last 10 years. Apart from the polymer Nafion®, which can be considered to be the standard, and fluoropolymers having a similar structure (Aciplex®, Flemion®, Hyflon®Ion), many polymers have been examined as proton conductors (W. Vielstich, A. Lamm, H. A. Gasteiger, Editors, Handbook of Fuel Cells, John Wiley & Sons, New York, 2003 and Hickner, M. A., H. Ghassemi, et al. (2004). “Alternative polymer Systems for proton exchange membranes (PEMs).” Chemical Reviews 104(10): 4587-4611).
For the two main uses of PEMs using hydrogen or methanol (in direct-methanol fuel cells (DMFCs)) as energy source (fuel), a membrane has to meet the following requirements (Hickner, M. A., H. Ghassemi, et al. (2004) loc. cit.): high proton conductivity, low electrical conductivity, low permeability to fuel and oxygen, low diffusive water transport or electroosmosis, high oxidative and hydrolytic stability, good mechanical properties in the dry state and (more importantly) in the hydrated state, low costs and processability to produce membrane-electrode assemblies (MEAs).
Many polymers have been proposed hitherto for PEMs. In the great majority of all examples, the proton transportability was achieved by introduction of sulfonic acid groups either subsequently by means of a suitable sulfonation method or during the synthesis by use of sulfonated monomers.
Thus, aliphatic polymers based on polystyrenes or polyvinyl alcohol in which the stability under PEM conditions was increased by partial fluorination have been proposed. A significantly larger number of sulfonated polymers from the class of aromatic polymers has been examined. Thus, polysulfones, polyether sulfones, polyether ether ketones, polyether ketone ketones, polyimides, poly(4-phenoxybenzoyl-1,4-phenylenes), polyethers (in particular those having tetraphenylphenylene units) and polybenzimidazoles, polybenzothiazoles, polybenzoxazoles, in each case in sulfonated form, have been described (Hickner, M. A., H. Ghassemi, et al. (2004), loc. cit. and Smitha, B., S. Sridhar, et al. (2005). “Solid polymer electrolyte membranes for fuel cell applications—a review.” Journal of Membrane Science 259(1-2): 10-26).
Sulfonated or carboxylated polymers are less suitable for proton conduction at temperatures above 100° C. since these groups lose water at high temperatures and the conductivity for protons is therefore significantly reduced. Phosphonium groups are significantly more stable under these conditions and have been favored for use at high temperatures (Stone, C, T. S. Daynard, et al. (2000). “Phosphonic acid functionalized proton exchange membranes for PEM fuel cells.” Journal of New Materials for Electrochemical Systems 3(1): 43-50; Jakoby, K., K. V. Peine-mann, et al. (2003). “Palladium-catalyzed phosphonation of polyphenylsulfone.” Macromolecular Chemistry And Physics 204(1): 61-67; Lafitte, B. and P. Jannasch (2005). “Phosphonation of polysulfones via lithiation and reaction with chlorophosphonic acid esters.” Journal of Polymer Science Part A-Polymer Chemistry 43(2): 273-286; Yamada, M. and I. Honma (2005). “Anhydrous proton conducting polymer electrolytes based on poly(vinylphosphonic acid)-heterocycle composite material.” Polymer 46(9): 2986-2992 and DE 101 48 131 A1 with further references).
Fully fluorinated, sulfonated polymers such as Nafion® and the like offer very high stability both toward oxidative attack and to hydrolytic decomposition. However, the formation of toxic fluoride ions in the form of HF (hydrofluoric acid), which result mainly from decomposition of free end groups of the polymer, is observed under use conditions (Curtin, D. E., R. D. Lousenberg, et al. (2004). “Advanced materials for improved PEMFC Performance and life.” Journal of Power Sources 131(1-2): 41-48). Distribution of the sulfonic acid groups in the incompatible, fluorinated polymer in this class of polymers results in a fine structure with formation of relatively large, water-filled clusters of sulfonic acid groups. When used as DMFC membrane, the membranes are swollen to a very high degree by the aqueous methanol solution and display an unacceptably high water and methanol transport to the cathode side.
Aliphatic, sulfonated polymers are more prone to oxidative attack than are aromatic polymers. Although fluorination at the susceptible points improves the stability, the formation of fluoride ions or fluorine radicals as toxic degradation product is possible. In the case of aromatic polymers, the required good mechanical properties are generally achieved by means of flexible ether groups in combination with rigid sulfoxide or ketone groups. As a direct consequence, the distribution of the proton-conducting sulfonic acid groups over the polymer chain is not uniform and can lead to poor proton transport when the polymer is swollen to only a small extent (Paddison, S. J. (2003) “Proton conduction mechanisms at low degrees of hydration in sulfonic acid-based polymer electrolyte membranes.” Annual Review of Materials Research 33: 289-319). Although proton transport is improved in the strongly swollen state, the disadvantages described in the case of Nafion®, e.g. high water transport, become apparent and the mechanical stability is greatly reduced.
In an article (Li, Q. F., R. H. He, et al. (2003) “Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 degrees C.” Chemistry of Materials 15(26): 4896-4915), Li et al describe solutions for fuel cell membranes operated at above 100° C. Polybenzimidazole membranes treated with phosphoric acid display particularly good stability. However, trifluoroacetic acid, for example, is used as solvent for producing these membranes (U.S. Pat. No. 5,716,727). This acid has a high vapor pressure at room temperature, is hazardous to health (R20)(Risk phrase 20 of the European Union Chemical Safety laws)) and poses a risk to bodies of water (R52/53). As an alternative, these membranes can also be produced from dimethylacetamide using 2% of LiCl and subsequent doping with phosphoric acid (U.S. Pat. NO. 5,525,436). However, in both cases, the phosphoric acid is not completely bound in the membrane and can migrate out during operation. Diffusion to the catalytic layer generally has an adverse effect on the catalytic reaction and damages the catalyst.